Performing measurements on fluid samples is desirable in many oil industry applications. Various methods exist for performing downhole measurements of petrophysical parameters of a geologic formation. Logging may be used in wellbores to make, for example, formation evaluation measurements to infer properties of the formations surrounding the borehole and the fluids in the formations. Typical logging tools may include electromagnetic (resistivity) tools, nuclear tools, acoustic tools, and nuclear magnetic resonance (NMR) tools, though various other types of tools for evaluating formation properties (also referred to as “formation parameters”) are also available. Early logging tools were run into a wellbore on a wireline cable after the wellbore had been drilled. Modern versions of such wireline tools may still be used extensively. However, as the demand for information while drilling a borehole continued to increase, measurement-while-drilling (MWD) tools and logging-while-drilling (LWD) tools have since been developed. MWD tools may typically provide drilling parameter information such as weight on the bit, torque, temperature, pressure, direction, and inclination. LWD tools may typically provide formation evaluation measurements such as resistivity, porosity, NMR distributions, and so forth. MWD and LWD tools may have characteristics common to wireline tools (e.g., transmitting and receiving antennas, sensors, etc.), but may be designed and constructed to endure and operate in the harsh environment of drilling.
The depth of detection provided by the logging tool is directly proportional to the distance between the transmitter and the receiver. As a result, most of the deep reading tools have very large distance between them. The LWD very deep resistivity basic tool configuration typically includes two or more independent drilling subs (one transmitter and one or more receivers) that are placed in a bottom hole assembly (BHA) among other drilling tools to allow large transmitter-receiver spacing. The basic measurements obtained with this tool consist of induction amplitudes at various frequencies, in order to allow detection of various formation layer boundaries with resistivity contrasts having a wide range of resistivities.
Multiple subs typically can communicate over a synchronous bus. Synchronous buses include a clock in the control lines and a fixed protocol for communicating that is relative to the clock. Synchronous buses have two disadvantages, however. First, the conventional wisdom is that every device on a bus must run at the same clock rate. Second, because of clock skew, distortion, and delay that can result from many factors, including line impedance, synchronous buses cannot be very long if they are high frequency. Therefore, it is very challenging for a synchronous sub bus to synchronize measurements over a long distance. Proper synchronization is desirable for getting good measurements and for avoiding undesired amplitude/phase measurement errors.
At least some of the ultra-deep reading tools that have been used by the industry achieve multi-coupling LWD measurements by a tilted antenna design. Special processing schemes in complex domain (consisting of real and imaginary part) are typically required to compensate such measurements and acquire signals with special sensitivity. As known in the art, there are various signal processing schemes to acquire LWD signals for various applications. However, since time synchronization errors impact measurement accuracy, the fundamental requirement for multi-antenna based tools is the complete and precise time synchronization of the tool to a single common reference time, and this becomes increasingly more challenging for longer antenna separation.
The ability to mitigate the effects of synchronization errors is of direct relevance to ultra-deep logging tools, particularly for real-time processing of measurements. Accordingly, there is continued interest in the development of calibration schemes capable of resolving synchronization issues.